The use of protective coatings on metal surfaces for improved corrosion resistance and paint adhesion characteristics is well known in the metal finishing arts. Conventional techniques involve pretreating metal substrates with a phosphate conversion coating and chrome-containing rinses for promoting corrosion resistance. The use of such chromate-containing compositions, however, imparts environmental and health concerns due to the toxic nature associated with chromium compounds.
As a result, chromate-free conversion coatings have been developed to overcome the need for chromate-containing compositions. Such chromate-free coatings are generally based on chemical mixtures that in some way will react with the substrate surface and bind to it to form protective layers.
Chromate-free conversion coatings typically employ a Group IVB metal such as titanium, zirconium or hafnium, a source of fluoride ion and a mineral acid to regulate the pH.
For example, U.S. Pat. No. 4,338,140 to Reghi discloses a conversion coating for improved corrosion resistance which includes zirconium, fluoride, and tannin compounds, and optionally phosphate ions. U.S. Pat. No. 5,759,244 discloses conversion coatings for metal substrates including a Group IVB metal in an acidic solution with one or more oxyanions, and which specifically excludes fluoride ions from the composition.
It has been suggested to include Group IA and/or Group IIA elements into such conversion coatings. For example, U.S. Pat. No. 5,441,580 to Tomlinson discloses the use of a Group IVB metal such as titanium, zirconium or hafnium, and Group IA metal such as potassium, and a source of fluoride ions, and U.S. Pat. No. 5,380,374 to Tomlinson discloses coatings based on such Group IVB metals including a Group IIA metal such as calcium at a concentration of 50 ppm to 1300 ppm. As is recognized in the art, for example in U.S. Pat. No. 5,964,928 to Tomlinson, coatings including Group IIA metals such as calcium generate considerable scaling from alkali metal precipitates, which may inhibit formation of the continuous metal oxide matrix. Such Group IIA metals are therefore generally used in lower concentrations. Also, as recognized in the U.S. Pat. No. 5,964,928 patent, such compositions including Group IA or Group IIA metals likely provide little if any long-range structure.
Accordingly, it would be desirable to provide a composition useful for coating metal substrates, particularly bare ferrous metals, which overcomes the environmental drawbacks of the prior art, which demonstrates excellent corrosion resistance and adherence of subsequently applied coatings, and which does not form a precipitate which may interfere with proper formation of the coating.